Mr. Peter Henderson says the ordinary span-roof is best. The walls are 4 ft. high, formed of locust or cedar posts. To the outside of these are nailed boards-rough hemlock will do, if appearances are not considered. To the boards is tacked the ordinary tarred paper used by roofers. Against the paper is again nailed the outer or weather boarding. This makes really a better wall for greenhouse purposes than an 8-inch one of brick, as we find that the extremes of temperature of the greenhouse-inside at 50°, and perhaps 10° below zero outside-very soon destroy an 8-inch solid brick wall, particularly if exposed to the north or west. A wall of wood constructed as above will last for twenty years, and be as good a protection as one of 8-inch brick. The roof is formed by the ordinary sashes, 6 ft. in length by 3 ft. in width, which can be bought ready made. Heat with a flue not more than 60 and not less than 30 ft. in length; if more, the flue would not heat it enough, and if less it would be likely to get too much heat. About 50 ft. by 11 is, we think, the best size of a greenhouse to heat with a flue. The flue should run all around the house-that is, it should start along under one bench, cross the end, and return under the other bench to the end where it begins, making the length of flue in a greenhouse of 50 feet about 110 feet long. It should have a "rise" in this length from the furnace of at least 18 in., to se cure a free draught. For the first 25 ft. of flue nearest the furnace it should be built of brick, forming an air-space inside of about 7 x 7 in. From this point (25 ft. from the fire) the flue should be formed of the ordinary drain-pipe cement or terra-cot-ta. The former is to be preferred, and that of 7 or 8 in. diameter is best. The cost of a greenhouse thus built in the vicinity of New-York, is about $6 per running foot-that is, one 50 ft. long by 11 ft. wide costs $300.